When I want to set a tag using `C-c C-q TAB' I get a iswitchb
completion

prompt, cause org-completion-use-iswitchb is t. I also use iswitchb's
virtual files feature (iswitchb-use-virtual-buffers set to t), which
includes files that aren't opened in a buffer, but are in the recentf

list. This is a cool thing when switching buffers (I barely use C-x
C-f

anymore and switch to the virtual buffer instead), but totally useless
and annoying for tab completion in org.
So when completing anything (files/tags/...) in org with iswitchb,
switchb-use-virtual-buffers should be let-bound to nil before.